1. Field of the Invention
The present invention relates to a phase frequency detector capable of improving in-band phase noise characteristics of a Phase Lock Loop or Phase Locked Loop (PLL), and more particularly to a phase frequency detector for acquiring linear output characteristics from all phase comparison ranges of a reference frequency and a division frequency, such that it can improve in-band characteristics of the PLL.
2. Description of the Related Art
Typically, a PLL is a requisite circuit required for a communication system to perform a variety of functions (e.g., frequency synthesis, and a clock or data recovery). Recently, the higher the operation speed of the communication system, the lighter the weight of the communication system. In order to implement the aforementioned characteristics, many developers are conducting intensive research into technology for low-voltage and low-power characteristics and the PLL capable of being stably operated at a high-frequency band.
FIG. 1 is a circuit diagram of a conventional PLL. A conventional PLL 10 will hereinafter be described with reference to FIG. 1.
Referring to FIG. 1, the PLL 10 divides a frequency created by a crystal (x-tal) oscillator (not shown) according to a channel step required for a reference divider (not shown), and generates a reference frequency (Fref). A phase frequency detector (PFD) 11 compares an output frequency of a Voltage Controlled Oscillator (VCO) 14 with a division frequency (Fdiv) created by the frequency divider 15 for dividing the output frequency of the VCO 14 at a predetermined ratio, such that it creates an Up or Down signal corresponding to a phase difference between the output frequency of the VCO 14 and the division frequency Fdiv.
A charge pump 12 receives the Up or Down signal from the PFD 11, creates a sourcing current by the Up signal, creates a sinking current by the Down signal, and provides a loop filter 13 with the sourcing or sinking current. In this case, an amount of current signals applied to the loop filter 13 may be adjusted by an additional signal received from an external part. The current signal applied to the loop filter 13 creates a voltage signal capable for generating an output frequency desired by the VCO 14, such that it controls the output frequency of the VCO 14. In other words, the VCO 14 creates an output frequency controlled by the output voltage of the loop filter.
The above-mentioned noise characteristics of the PLL 10 are classified into first noise characteristics generated by the PFD 11 and second noise characteristics created by the VCO 14. If a transfer function is calculated under the above-mentioned situation of the first or second noise characteristics, the following characteristics may occur. In more detail, transfer characteristics of noise created by the PFD 11 are inversely proportional to a gain of the PFD 11, and are proportional to a division ratio of the frequency divider 15. In the meantime, transfer characteristics of noise created by the VCO 14 are also associated with characteristics of the loop filter 13. Namely, the noise created by the PFD 11 is equal to in-band phase noise of the PLL 10, and the noise created by the VCO 14 is equal to out-band phase noise of the PLL 10.
As described above, a factor for determining the in-band phase noise within a bandwidth designed for optimum characteristics of the PLL is indicative of the noise created by the PFD. The above-mentioned noise created by the PLL indicates the gain and linearity of the PFD.
Therefore, there must be newly developed an improved technology capable of solving nonlinearity of the PFD to improve in-band phase noise characteristics of the PLL.